This invention relates to a method and system for feeding comminuted cellulosic fibrous material to a treatment vessel, such as a continuous digester. The invention simplifies and dramatically reduces the number of components needed when compared to the existing art.
U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025 and 5,766,418 introduced the first real breakthroughs in the art of feeding comminuted cellulosic fibrous material to a treatment vessel in over forty years. These patents and the application disclose several embodiments, collectively marketed under the trademark Lo-Level® feed system by Andritz Inc. Inc. of Glens Falls, N.Y., for feeding a digester using a slurry pump, among other components. As described in these patents and application, using such a pump to feed a slurry to a high-pressure transfer device dramatically reduces the complexity and physical size of the system needed, and increases the ease of operability and maintainability. The prior art systems employing a high-pressure transfer device, for example a High-Pressure Feeder as sold by Andritz Inc., but without such a pump, are essentially unchanged from the systems sold and built since the 1940s and 1950s.
The present invention relates to an even more dramatic improvement to the methods and systems disclosed in the above-mentioned patent and applications. The present invention actually eliminates the need for transfer devices, such as a High-Pressure Feeder, by using high-pressure pumping devices to transfer a slurry of comminuted cellulosic fibrous material directly to a digester.
The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180° C. Since the aqueous chemicals used to treat the material would boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel under pressures of at least about 10 bars gauge (approximately 150 psi gauge). In order to maintain this pressure, especially when performing a continuous pulping process, special accommodations must be made to ensure that the pressure is not lost when introducing material to the pressure vessel. In the prior art this was accommodated by what is known in the art as a “High-Pressure Feeder”. This feeder is a specially-designed device containing a pocketed rotor which acts as a means for transferring a slurry of material from a low pressure to a high pressure while also acting as a valve for preventing loss of pressure. This complicated and expensive device has long been recognized as an essential component for introducing slurries of comminuted cellulosic material to pressurized vessels, typically at elevated temperatures, especially to continuous digesters.
According to the invention a system which replaces the High-Pressure Feeder—which has been recognized for over forty years as being essential to continuous digesting—is provided, greatly simplifying construction of a pulp mill. The system includes first and second high pressure slurry pumps coupled in series and which pumps replace a conventional High Pressure Feeder. The first high pressure slurry pump is a conventional single vane, screw pump and the second pump is a centrifugal pump.
According to one aspect, a system for producing chemical cellulose pulp from comminuted fibrous cellulose material, such as wood chips, comprises the following components: A steaming vessel in which comminuted fibrous cellulose material is steamed to remove the air therefrom. A superatmospheric pressure vertical treatment vessel having an inlet for a slurry of comminuted cellulose fibrous material at a top portion thereof and an outlet at a bottom portion thereof. And, pressurizing transfer means for pressurizing a slurry of material from the steaming vessel and transferring it to the treatment vessel inlet, the pressurizing transfer means consisting of one or more high pressure slurry pumps located below the top portion of the treatment vessel.
The one or more pumps preferably comprises first and second high pressure slurry pumps connected in series and each having a pressure rating, an inlet and an outlet, the first pump inlet operatively connected to the steaming vessel, the first pump outlet operatively connected to the second pump inlet, and the second pump having a higher pressure rating than the first pump. The first high pressure slurry pump may be a single vane screw pump. The second high pressure slurry pump may be a helical screw centrifugal pump, double-piston solids pumps, or other similar conventional pumping devices that are capable of pressurizing a slurry having a relatively high percentage of solids to (in one or more stages) a pressure of at least about 5 bar gauge. The pressurizing and transferring may also be effected by an one or more eductors, of conventional construction, driven by a pressurized fluid supply, such as supplied by conventional centrifugal pump.
One typical unit of measure that indicates the relative amount of solids in a slurry containing solids and liquid is the “liquid-to-solids ratio”. In this application, this ratio is the ratio of the volume of liquid being transferred to the volume of cellulose, or wood, material being transferred. Typical conventional centrifugal liquid pumps are limited to pumping liquid having a solids content of at most 3%. This 3% solids content corresponds to a liquid-to-solids ratio of about 33. In the slurry pumps of this invention, the liquid-to-solids ratio of the slurry being pumped is typically between 2 and 10, preferably between 3 and 7, and most preferably between 3 and 6. In other words, the slurry pumps of this invention transfer slurries having a much greater solids content than can be handled by a conventional pump.
A liquid return line may be provided from the top portion of the treatment vessel, containing liquid separated from the slurry at the top of the treatment vessel (preferably a continuous digester). The return line may be operatively connected to an inlet or outlet of one of the slurry pumps, either directly or indirectly. Preferably the liquid return line is connected to a pressure reduction means for reducing the pressure of liquid in the return line before the liquid passes to the inlet or outlet of the slurry pump. The pressure reduction means may take a variety of forms, such as a flash tank and/or a pressure control valve in the return line, or other conventional structures for effectively reducing the pressure of liquid in a line while not adversely affecting the liquid. Where a flash tank is utilized the liquid outlet from the flash tank is connected to the inlet to the first slurry pump, and the steam produced by the flash tank may be used in the steaming vessel.
Alternatively, the pressure reduction may be effected, or even avoided, by using an eductor which uses the pressurized return line liquor as its source of pressurized fluid. An eductor may be used in place of or in conjunction with one or more of the slurry pumps, or other devices, to transfer slurry to the digester.
A conventional chute, as well as other optional components, is preferably connected between the steaming vessel and the at least one slurry pump, the steaming vessel being located above the chute and the chute above the at least one slurry pump. The at least one slurry pump is typically located a distance at least 30 feet (about 10 meters) below the top of the digester, and typically more than about 50 feet (about 15 meters) below.
When the high pressure transfer device is eliminated it is desirable to utilize other mechanisms to retain one of the functions of the high pressure transfer device, namely providing pressure relief prevention should an aberrant condition occur, the high pressure transfer device typically preventing backflow of liquid from the digester into the feed system. Pressure relief preventing means according to the present invention are preferably distinct from the at least one slurry pump, although under some circumstances the inlets to or outlets from the slurry pumps may be constructed in a manner so as to provide pressure relief prevention. The pressure relief preventing means may comprise an automatic isolation valve in each of the slurry conduits transferring slurry from the pumps to the top of the treatment vessel and the return line from the treatment vessel, a conventional controller being provided connected to the isolation valves and operating the isolation valves in response to the pressure sensed by a pressure sensor associated with the slurry conduit feeding slurry to the top of the treatment vessel. The pressure relief preventing means may also comprise a check valve in the slurry conduit, and/or a variety of other valves, tanks, sensors, controllers, or like fluidic, mechanical, or electrical components which can perform the pressure relief preventing function.
The system may also comprise means for augmenting the flow of liquid to the inlet to the second slurry pump, or to any pump or transfer device, such as a liquid line having liquid at a pressure below the pressure at the second slurry pump inlet, a conduit between the liquid line and the inlet, and a liquid pump in the conduit. The liquid line may be the return line from the treatment vessel, and the conduit may be connected directly to the return line. The liquid return line may be connected to a flash tank as described above, and the conduit may be connected to the flash tank liquid outlet.
According to another aspect, a method of feeding comminuted cellulosic fibrous material to the top of a treatment vessel is provided. The method comprises the steps of: (a) Steaming the material to remove air therefrom and to heat the material. (b) Slurrying the material with a cooking liquor to produce a slurry of liquid and material. And, (c) pressurizing the slurry to a pressure of at least about 5 bar gauge at a location below the top of the treatment vessel (e.g. at least thirty feet below, preferably at least fifty feet below), and transferring pressurized material to the top of the treatment vessel, the pressurizing step consisting of acting on the slurry with one or more high pressure slurry pumps.
The method may comprise the further steps of: (d) returning liquid separated from the slurry at the top of the treatment vessel to the at least one pump; and (e) sensing the pressure of the slurry while being transferred to the top of the treatment vessel, and shutting off the flow of slurry to the top of the treatment vessel and the return of liquid from the top of the vessel if the sensed pressure drops below a predetermined value. There also may be the step (f) of flashing the liquid while returning in the practice of step (d) to produce steam, and using the steam in the practice of step (a).
In an additional embodiment, the concept of transferring a slurry of chips is extended back to the point where chips are introduced to the mill, that is, the Woodyard. Conventional pulp mills receive their supply of cellulose material, typically hardwood and softwood but other forms of cellulose material as described above may be handled, in various forms. These include as sawdust, as chip, as logs, as long de-limbed trees (that is, “long wood”), or even as complete trees (that is, “whole trees”). Depending upon the source of cellulose of the “wood supply”, the wood is typically reduced to chip form so that it can be handled and treated in a pulping process. For example, devices known as “chippers” reduce the long-wood or logs to chips that are typically stored in open chip piles or chip silos. This receipt, handling, and storage of the chips is performed in an area of the pulp mill referred to as the “Woodyard”. From the Woodyard the chips are typically transferred to the pulp mill proper to initiate the pulping process.
In conventional Woodyards, the chips are stored in silos from which the chips are discharged, typically by means of a rotating or vibrating silo discharge device, to a conveyor. This conveyor is typically a belt-type conveyor which receives the chips and transfers them to the pulping treatment vessels. Since the Woodyard is typically at a distance from the pulping vessels, this conveyor is typically long. Such conveyors may have a length of up to one-half mile. In addition, treatment systems that do not employ the Lo-Level® feeding system, as marketed by Andritz Inc. and described in U.S. Pat. Nos. 5,476,572, 5,622,598, 5,635,025 and 5,766,416, require that the conveyor be elevated, typically to a height of at least 100 feet, in order to feed the chips to the inlet of the first pulping vessel. These conveyers, and the structures that support them, are very expensive and contribute a significant cost to the cost of a digester feed system.
In another embodiment, the concept of transferring a slurry of chips is extended back to the Woodyard. A preferred embodiment of this invention consists of a method of transferring comminuted cellulosic fibrous material to a pulping process, consisting of the following steps: (a) Introducing untreated chips to a first vessel. (b) Introducing slurrying liquid to the first vessel to create a slurry of material and liquid. (c) Discharging the slurry from the vessel to the inlet of at least one pressurizing and transferring device. (d) Pressurizing the slurry in the pressurizing and slurrying device and transferring the slurry to a treatment vessel.
The first vessel is typically a chip storage silo or bin. This bin preferably has a discharge having one-dimensional convergence without agitation or vibration, such as a DIAMONDBACK® bin as described in U.S. Pat. No. 5,000,083, though agitation or vibration may be used. This bin may also have two or more outlets which feed two or more transfer devices. This vessel may also be operated at superatmospheric pressure, for example at 0.1 to 5 bar. If the vessel is operated at superatmospheric pressure some form of pressure isolation device must be located at the inlet of the vessel to prevent the release of pressure. This device may be a star-type isolation device, such as a Low-pressure Feeder or Air-lock Feeder as sold by Andritz Inc., or a screw-type feeder having a sealing capacity as described in U.S. Pat. No. 5,766,416.
The slurrying liquid may be any source of liquid available in the pulp mill, including fresh water, steam condensate, kraft white, black, or green liquor or sulfite liquor or any other pulping-related liquid. This liquid may be a heated fluid, for example, hot water or steam, having a temperature of between 50 and 100° C. If the vessel is a pressurized vessel, liquid temperatures of over 100° C. may be used. Though not essential, this liquid may contain at least some active pulping chemical, for example, sodium hydroxide (NaOH), sodium sulfide (Na2S), polysulfide, anthraquinone or their equivalents or derivatives or surfactants, enzymes or chelates, or combinations thereof.
The pressurizing and transferring device of steps (c) and (d) is preferably a slurry pump, or pumps, but many other pressurizing and transferring devices may be used such as the piston-type solids pump or a high-pressure eductor. Preferably, more than one pressurizing and slurrying pump is used to transfer the slurry. These may be two or more slurry pumps, or any combination of slurry pump, piston-type pump, or eductor. This transfer system may also include one or more storage or surge tanks as well as transfer devices. Preferably, the one or more transfer devices include at least one device having de-gassing capability so that undesirable air or other gases may be removed from the slurry. Also, during transfer, the chips may be exposed to some form of treatment, for example, de-aeration or impregnation with a liquid, preferably a liquid containing pulping chemicals, such as those described above. The slurry may also be exposed to at least one pressure change or fluctuation during transfer, for example, such that the pressure of the slurry is varied from a first pressure to a second, higher pressure, and then optionally to a third pressure which is lower than the second pressure. As described in U.S. Pat. Nos. 4,057,461 and 4,743,338 varying the pressure of a slurry of chips and liquor improves the impregnation of the chips by the liquor. This pressure pulsation may be achieved by varying the outlet pressure of a set of transfer devices in series, or by controlled depressurization of the slurry between pumping.
In another embodiment, the material need not encounter liquid in the vessel, but may have liquid first introduced to it by means of an eductor located in or below the outlet of the vessel. This liquid is preferably pressurized so that the material and liquid form a pressurized slurry of material and liquid.
The treatment vessel of step (d) may typically be a steaming vessel as described above, preferably a DIAMONDBACK® steaming vessel. The vessel may also be a storage or surge tank in which the material may be stored prior to treatment. Since the transfer process may require excess liquor that is not needed during treatment or storage, some form of de-watering device may be located between the transfer device and the treatment vessel. One preferred dewatering device is a Top Separator, as sold by Andritz Inc. This Top Separator may be a standard type or an “inverted” Top Separator. This device may be an external stand-alone-type unit or one that is mounted directly onto the treatment vessel. An In-line Drainer, also sold by Andritz Inc., may also be used for the dewatering device. Preferably, the liquid removed from the slurry by means of the dewatering device is returned to the first vessel or to the transfer devices to act as the slurring liquid. This liquid may also be used where ever needed in the pulp mill. This liquid may be heated or cooled as desired. For example, this liquid may be heated by passing it in indirect heat exchange relationship with any heated liquid stream, for example, a waste liquid stream having a temperatures greater than 50° C. This liquid will also typically be pressurized using one or more conventional centrifugal liquid pumps.
In one preferred embodiment the treatment vessel of step (d) is a steaming vessel which feeds one or more transfer devices as described above. Though this system is preferably used in conjunction with a feed system not having a conventional High-pressure Feeder, this system may also be used with a feed system having a High-pressure Feeder.
The method and apparatus for feeding chips from a distant location, for example, a Woodyard, to a pulping process is not limited to chemical pulping processes, but may be used in any pulping process in which comminuted cellulosic fibrous material is conveyed from one location to another. The pulping processes that this invention is applicable to include all chemical pulping processes, all mechanical pulping processes, and all chemi-mechanical pulping or thermal-mechanical pulping processes, for either batch or continuous treatment.